Biologia 70/8: 1019—1025, 2015 Section Cellular and Molecular Biology DOI: 10.1515/biolog-2015-0130 Mitochondrial structures during seed germination and early seedling development in Arabidopsis thaliana* José L. Rodríguez 1,2,3 **, Juana G. De Diego 1 , Francisco D. Rodríguez 1 & Emilio Cervantes 2 1 Departamento de Bioquímica y Biología Molecular, Edificio Departamental, Campus Miguel de Unamuno, Universidad de Salamanca, 37007 Salamanca, Spain 2 IRNASA-CSIC, Cordel de Merinas, 40, 37080 Salamanca, Spain 3 Plant Developmental Genetics, Institute of Biophysics, The Czech Academy of Sciences, v.v.i., Královopolská 135, 612 65 Brno, Czech Republic; e-mail: rodriguez@ibp.cz Abstract: Mitochondrial morphology and evolution have been observed during seed germination and early seedling de- velopment in Arabidopsis thaliana line 43a9 (ecotype Columbia) expressing green fluorescent protein in these organelles. Fluorescence, confocal and electronic microscopy images reveal that mitochondrial development goes through different stages, and that the organelle structure varies with cell types during these processes. Mitochondria develop from larger, isodiametric structures pre-existent in the dry seed called promitochondria. After germination, variations in mitochondrial morphology occur synchronously with cell differentiation and cell division in the course of early root development. Some promitochondria develop into intermediate structures resembling the syncytial organelles. These structures have been de- scribed in certain plants under hypoxia as intermediates for the formation of mature mitochondria. On the other hand, other promitochondria temporarily remain in the cells of the root apex. Key words: Arabidopsis; confocal microscopy; germination; Mitotracker; promitochondria. Abbreviations: CycB1, cyclin-dependent protein kinase B1; GFP, green fluorescent protein; TEM, transmission electron microscope. Introduction Arabidopsis thaliana has unique advantages as a model system in plant biology. In addition to a small genome, from which a large amount of genomic and expressed sequence information is available, other important ad- vantages include a small seed size and reduced number of cells in the early stages of development. Together with a growing variety of available research resources, these features make Arabidopsis an excellent system to study cell differentiation during seed germination and early root development (Koornneef & Meinke 2010). In early developmental stages, the Arabidopsis root is a system situated at the interface between cell and organ- ism biology. Many Arabidopsis stock lines containing useful gene constructions for the analysis of differenti- ation, such as those expressing green fluorescent pro- tein (GFP) in different organelles, structures and cell types, are now available. In these lines, the cells and sub-cellular structures may be observed in vivo by con- focal microscopy and hence the diverse differentiation processes may be correlated (Logan & Leaver 2000). Seed germination is a rapid process in Arabidopsis. It includes important changes in gene expression as well as in morphology (de Diego et al. 2006, 2007; Cervantes et al. 2010; Martín et al. 2014). After cold stratification and exposure of seeds to light under appropriate tem- perature, radicle emergence throughout the testa (ger- mination) occurs in approximately 20 h. Germination is the product of cell elongation. The first cell divisions in the root cells are observed after germination (Bar- rôco et al. 2005). From the time when radicle emergence occurs, changes in cell structure and metabolic status include cellular re-organization and organelle differen- tiation. Once the root has reached a certain size, dif- ferentiation of cell types from the quiescent centre has been described (Schiefelbein et al. 1997). Mitochondria are very dynamic organelles (Wel- chen et al. 2014). They are present in the dry seeds (Yoo 1970; Attucci et al. 1991) being numerous in particular cell types, for example in the aleurone layer of cere- als (Oparka et al. 1981). Furthermore, in dry seeds and * Electronic supplementary material. The online version of this article (DOI: 10.1515/biolog-2015-0130) contains supple- mentary material, which is available to authorized users. ** Corresponding author c 2015 Institute of Molecular Biology, Slovak Academy of Sciences